In rotating anode type of X-ray tubes, it is desirable to provide a means to assure that the anode has attained a selected rotational speed prior to the electron bombardment of the anode and the generation of X-rays. Certain prior art systems utilize time dependent apparatus for accelerating the anodes. In such prior art devices, a boost or acceleration signal is applied to the anode to start the anode rotating, and the generation of the X-rays and the bombardment of the anode is delayed for a fixed period of time to enable the anode to reach a selected rotational speed before the X-rays bombard on the anode. However, in boosting the speed of the anode to the desired speed, the prior art relies on a boost signal which is applied to the associated stator for a pre-selected fixed time and there is no other control to assure that the anode is actually rotating at the selected desired speed at the termination of the time period of the boost signal.
In prior art systems, the rotating anodes of the tubes are generally driven with split phase motors and the motors are operated from power sources capable of providing either a 60 Hz or 180 Hz AC control power to enable operation at rotational speeds of approximately 3,600 RPM or 10,800 RPM, respectively. Such two speed drive systems are normally preferred over single speed drives because the high speed drive provides greatly improved X-ray tube operating ratings and loadings.
As disclosed in U.S. Pat. No. 3,641,408, to provide a source of both 60 Hz and 180 Hz power supply, two separate starter systems have been employed wherein the anode system obtains a first source of power at a line frequency power and another source of power at a multiple of the line frequency. Further, each drive system requires means for providing either a normal running voltage and a relatively higher boost voltage. The boost voltage is used to accelerate the anode to obtain the desired speed of rotation in the shortest possible time, whether it be the low speed 3,600 RPM rotation or the 10,800 RPM rotation.
As an X-ray tube is used repeatedly, the associated mechanical and support assembly of the rotating anode heats up and more power is dissipated. As the tube housing gets hotter, the associated stator loses efficiency since its resistance goes higher, while the inductance remains fixed and causes more power to be dissipated in the form of heat in the windings and less power is transmitted to the anode rotor. Thus, it has been found that after the tube gets hot, most stators do not receive the required amount of energy to drive the anode to attain the desired running speed.
Another problem of rotating anode tubes is the vibration caused by the mechanical resonance of the anode system. In presently available tubes, the mechanical resonance of the system is at approximately 6,000 RPM. Accordingly, it is desirable that the anode be accelerated and decelerated or braked through this mechanical resonance point in a minimum of time to reduce any wear and damage caused by vibration as the anode speed goes through this point.